ABSTRACT
Neurotransmitters are the chemical agents released from the presynaptic bulb into the synaptic cleft. They are sometimes referred to as the primary messenger, since they occur as free chemicals that move across a space (here it is the cleft) and cause a change in another part (in this case, the postsynaptic membrane). Another term used for neurotransmitters is ligand, meaning an agent that binds to a receptor and in so doing changes that receptor. The main change that occurs in receptors on binding of a ligand is an allosteric effect, i.e. a change of receptor shape. This change then has a further effect, the nature of which depends on which receptor is considered; it may be either within the membrane or beyond it in the cytoplasm of the cell.
